I spent a little time pursuing the course of using the Schema.org module to code the RDF in the output and there are two major problems I'm encountering with that approach. They're detailed below, but because of them we may need to abandon this track and pursue another approach.

Thanks, for tracking this, @scor. I did a bit of testing over the weekend and it looks like converting to use the node display works for this use case.

On the Distribution field, it's set up as a custom field type which functionally provides three separate fields (format, size, and accessurl). Each of these need separate rdfa attributes (http://project-open-data.github.io/metadata-resources/#common_core_requi...) but since they're under the custom field type the field editing UI only gives us access to the one field. I don't think that's really your module's problem to handle, but it is an issue that we need to address.

re distribution field, since it is part of your module/distro, it would be easy to add RDFa markup directly on the field formatter. I'm at DrupalCon this week, but I can take a look at it next week....